When studying the dependence of biological activities on temperature, temperature-gradient incubators are useful devices in a variety of ways. In nearly all solutions described in literature, a temperature gradient is produced by heating one end of an elongated metal body to a constant temperature. The other end is either subjected to free cooling or cooled to a constant temperature. By virtue of heat conduction, the metal body generates a more or less linear temperature gradient which can be utilized for studying the growth and activity of microbes at continuously varying temperatures. It is essential to produce a uniform temperature gradient and a complete temperature-gradient linearity over the entire area of a temperature-gradient plate as non-linearity results in visible curving of isotherms parallel to the longer side of a gradient plate and causes errors in the determination of temperature ranges for microbe cultures.
The temperature-gradient incubators described in literature and intended for microbiological application include primarily two types of devices. One type of devices are intended for growing microbes in fluid cultures, the other for growing in agar jelly or some other "solid" culture medium. In the latter type, it is possible to produce a stepless or continuous temperature gradient. A typical characteristic in commercially available equipment is that temperature varies along the greatest dimension of a metal body whereby temperature distribution will be sufficiently stabilized. If the culture capacity should be increased in lateral direction, either temperature distribution will be difficult to control or the equipment will large and bulky provided that the longitudinal direction will still be maintained as gradient direction. In general, the capacity of prior known temperature-gradient incubators ranges from a single culture to a few cultures per run.
The publication Can. J. Microbiol., vol. 19, No. 9, 1973 pp. 1161-1165 discloses a temperature-gradient incubator wherein the ends of circulation passages are connected by means of flow channels, built inside the corners of one end of a massive temperature-gradient plate, said channels being covered with plates. Between the cover plates and the end of gradient plate is a thin rubber seal. A result of this construction is that the transfer of heat within the corner regions of a gradient plate between the medium and the gradient plate and also between the gradient plate and the environment is different from the transfer of heat within the central region of a gradient plate. The same is true also within those corner regions of a gradient plate in which a medium is introduced into the gradient plate passages since the supply tubes are connected directly to the ends of the passages. In order to achieve an improved linearity for temperature gradient isotherms, this prior known gradient plate has a substantial thickness and the heat transfer elements are mounted along the short sides of a gradient plate. Thus, the linearity of isotherms is accomplished at the cost of high energy consumption and limited testing capacity.
An object of the invention is to provide an improved temperature-gradient incubator with a construction as simple as possible, which is capable of achieving an improved linearity for isotherms without resorting to great plate thickness and having to have a low testing capacity.